The present invention relates to a technique of improving the safety and reducing a current consumption in a battery device and in a battery state monitoring circuit capable of calculating the remaining amount of a secondary battery such as lithium ion secondary battery.
As a conventional battery state monitoring circuit, there has been known a device shown in a circuit block diagram of FIG. 2. This structure is disclosed in, for example, Japanese Patent Application Laid-open No. Hei 9-312172, xe2x80x9cBattery Pack, Charger, Charging System and Charging Method.xe2x80x9d This relates to a battery device which is called xe2x80x9cSmart Battery Systemxe2x80x9d.
The lithium ion secondary battery used in the smart battery system requires an over-charge protecting circuit since it is not provided with a self-protecting operation unlike a nickel/cadmium battery. A battery device 22A is equipped with a battery voltage monitoring circuit 4A for detecting a battery voltage and a switch element 12 for stopping the charging from an external charger 21.
The battery device 22A controls the indication of the remaining amount of each of secondary batteries 7 to 10, the stop of charging, etc. A microcomputer 6A is inputted with an output voltage V3a of a current monitoring circuit 3A and an output voltage V4a from a battery voltage monitoring circuit 4A. The microcomputer 6A is capable of calculating the charging currents and the discharging currents of the secondary batteries 7 to 10 and the respective battery voltages based on the inputted voltages V3a and V4a to calculate the remaining amounts of the secondary batteries 7 to 10. Also, the microcomputer 6A controls the on/off operation of the switching elements 12 and 13 in accordance with the state (normal, over-charging, over-discharging and over-current) of the secondary battery. That is, the microcomputer 6A controls the protecting function (over-charging protection, over-discharging protection and over-current protection) of the battery device 22A.
In the battery device 22A, a battery state monitoring circuit 14A has a total battery voltage VBa of the secondary batteries 7 to 10 as a power supply.
The microcomputer 6A is inputted with an output V1a of a regulator 1A as a power supply of the microcomputer 6A. since the total battery voltage VBa of the secondary batteries 7 to 10 changes in accordance with the status of a load 20 connected between a plus terminal 15 and a minus terminal 17 of the battery device 22A, the total battery voltage Vba is regulated to a constant voltage V1a such as 3.3 V or 5.0 V by a regulator 1A and then supplied to the microcomputer 6A.
The microcomputer 6A is inputted with an output current I3a (in the for m of a voltage V3a) of the current monitoring circuit 3A and calculates the remaining amount of the battery in accordance with the output current I3a. In general, the output of the current monitoring circuit 3A is an analog value which needs to be converted into a digital value in order to calculate the remaining amount of battery by the microcomputer 6A. For that reason, an A/D convertor circuit is installed in the microcomputer 6A.
As described above, because the power supply of the battery state monitor circuit 14A is supplied from the secondary batteries 7 to 10, an electric power is consumed from those secondary batteries.
Also, the charging and discharging voltage/current from the secondary batteries are not constant but largely change in a short period of time in accordance with the status of the load 20 or the status of the charger 21. For that reason, in order that the battery state monitoring circuit 14A monitors the charging and discharging current from the secondary batteries 7 to 10 with a higher precision, the operating frequency of the A/D convertor installed in the microcomputer 6A needs to be set as high. As the operating frequency of the A/D convertor is set as high, the current consumption of the microcomputer 6A is further increased, to thereby further increase the consumption of a power from the secondary batteries.
Under the above circumstances, there has been proposed a method of stopping the A/D convertor within the microcomputer 6A and the charging/discharging current monitoring function as a technique in which, in the battery state monitoring circuit 14A, when the charging and discharging current of the secondary batteries 7 to 10 is small, for example 0 to 10 mA, as compared with the battery capacity of the secondary batteries, the current consumption of the entire circuit including the charging and discharging monitoring circuit 3A is reduced, to thereby suppress the power consumption of the secondary batteries 7 to 10.
In this state, the consumed current of the battery state monitoring circuit 14A is reduced down to 1/10 to 1/1000. In this way, the state where the consumed current is reduced is called xe2x80x9cpower save modexe2x80x9d.
However, certain conditions are required when the operation is returned from the power save mode to the normal operation. A function of returning the battery state monitoring circuit 14A to the normal operation is called xe2x80x9cwake upxe2x80x9d. The condition for waking up is set, for example when xe2x80x9cthe charging and discharging current of the secondary batteries 7 to 10 exceeds 1 ampere.xe2x80x9d Under that condition, the minus pole voltage V21a of the charger 21A is a small value lower than 50 mV.
In order to detect such a fine voltage, the current monitoring circuit 3A must make an offset voltage small so that the fluctuation of the detected wake-up voltage is reduced. Also, since the current monitoring circuit 3A operates even in the power save mode, the current consumption must be reduced. The current monitoring circuit of this type is complicated in structure and cannot be realized by a simple and inexpensive circuit.
In order to solve the above problems, according to the present invention, a filter is disposed between a sense resistor terminal and a control circuit, which is capable of reducing the current consumption of the entire circuit when the charging and discharging current value becomes small and detecting the current when the charging and discharging current becomes large.
The circuit is structured in such a manner that only the current monitoring circuit operates when the charging and discharging current value becomes small, and the sense resistor terminal and an input terminal of the control circuit current are connected with a capacitor so that all the functions for detecting the remaining amount of a battery start to be executed when the charging and discharging current changes.